Implantable cardiac stimulation devices, such as pacemakers, are often configured to be used in conjunction with an external programmer that allows a physician to program the operation of the implanted device to, for example, control the specific parameters by which the implanted device detects a heart arrhythmia and responds thereto. For instance, the programmer may allow the physician to specify the sensitivity with which the cardiac stimulation device senses electrical signals within the heart and to further specify the amount of electrical energy to be employed for pacing the heart in circumstances where expected heart signals are not sensed. If the cardiac stimulation device includes a physiological sensor, the programmer may also permit the physician control in the manner by which the device responds to signals generated by the sensor. Examples of sensors include minute ventilation sensors for detecting respiration signals, temperature sensors, oxygen blood saturation sensors and the like.
In many cases, a sensor is used for rate-responsive pacing whereby the cardiac stimulation device determines the rate at which the heart of the patient is to be paced based on the conditions detected by the sensor. In one specific example, the pacing rate is determined based on the impedance signal measured by a minute ventilation sensor. FIG. 1 illustrates a minute ventilation response graph displayed by an external programmer, which relates minute ventilation values to pacing rates. In use, the external programmer generates a bi-linear minute ventilation response graph 10 for display to the physician based on an initial set of control parameters. The physician can modify the set of control parameters and display the resulting adjusted bi-linear response function 20. The physician then selects either the initial set or the modified set of control parameters, which are transmitted to the implantable device for use therein. Thereafter, the stimulation device uses the selected control parameters to automatically calculate a pacing rate based on the calculated minute ventilation signals as detected by the sensor, then the device paces the heart at the calculated sensor indicated rate. Assuming the rate-response control parameters have been properly selected, the stimulation device can approximate a healthy sinus node response by increasing the pacing rate as the level of physical exertion of the patient increases as detected by increased ventilation in the lungs.
Although rate-responsive pacing has proven to be effective for mimicking the response of a healthy sinus node, room for improvement remains. In the conventional system summarized above, the rate-response control parameters that the physician can select only define bi-linear rate-response functions, i.e. the resulting rate response graphs of FIG. 1 are defined by two straight segments. It appears, however, the actual relationship of minute ventilation to sinus rate is curvilinear in most patients. As such, a physician using a programming system limited to bi-linear rate-responsive pacing functions may not be able to program the device to mimic the response of a healthy sinus node as closely as might be preferred. Accordingly, it would be highly desirable to provide an improved technique for programming implantable cardiac stimulation devices, which permits more flexible programming of rate-responsive pacing functions, such as minute ventilation functions, and it is to this end that aspects of the invention are directed.
If two or more sensors are used, the implantable cardiac stimulation device typically combines the pacing rates determined from the separate sensors to yield a single pacing rate. For example, if the device includes both a minute ventilation sensor and an activity sensor, the device may determine one pacing rate based on the minute ventilation and another rate based on the activity level then combine the rates to yield a final pacing rate for use in pacing the heart. Typically, the microcontroller of the cardiac stimulation device is programmed to combine the pacing rates from the sensors in a fixed predetermined manner regardless of the current level of exertion of the patient. For example, the microcontroller may be programmed to average the two pacing rates to yield the final pacing rate. Hence, the two sensors contribute equally to the determination of the final pacing rate regardless of the current level of exertion. However, one sensor may be more effective than another at different levels of exertion. It would be preferable, therefore, to permit the physician to select the relative contributions of multiple sensors for different levels of exertion and it is to this end that other aspects of the invention are directed.
Also, many conventional implantable cardiac stimulation devices permit programming of only a single rate-responsive pacing function for each sensor. In some cases, however, it would be preferable to permit programming of different rate-responsive pacing functions depending upon the general condition of the patient. For example, if the patient is in a prolonged state of bed rest, then a rate-response function programmed with the expectation that the patient will be more active may be too aggressive resulting in unnecessarily high heart rates. In contrast, if the patient is generally active, then a rate-response function programmed with the expectation that the patient will be primarily at bed rest may be too passive resulting in generally lower heart rates than optimal. Accordingly, it would be preferable to permit the programming of several rate-responsive pacing functions for each sensor for use under different conditions and it is to that end that still other aspects of the invention are directed.